Mental rotation of hands and objects in ageing and Parkinson’s disease: differentiating motor imagery and visuospatial ability

Motor imagery supports motor learning and performance and has the potential to be a useful strategy for neurorehabilitation. However, motor imagery ability may be impacted by ageing and neurodegeneration, which could limit its therapeutic effectiveness. Motor imagery can be assessed implicitly using a hand laterality task (HLT), whereby laterality judgements are slower for stimuli corresponding to physically more difficult postures, as indicated by a “biomechanical constraint” effect. Performance is also found to differ between back and palm views of the hand, which may differentially recruit visual and sensorimotor processes. Older adults and individuals with Parkinson’s disease (PD) have shown altered performance on the HLT; however, the effects of both ageing and PD on laterality judgements for the different hand views (back and palm) have not been directly examined. The present study compared healthy younger, healthy older, and PD groups on the HLT, an object-based mental rotation task, and an explicit motor imagery measure. The older and PD groups were slower than the younger group on the HLT, particularly when judging laterality from the back view, and exhibited increased biomechanical constraint effects for the palm. While response times were generally similar between older and PD groups, the PD group showed reduced accuracy for the back view. Letter rotation was slower and less accurate only in the PD group, while explicit motor imagery ratings did not differ significantly between groups. These results suggest that motor imagery may be slowed but relatively preserved in both typical ageing and neurodegeneration, while a PD-specific impairment in visuospatial processing may influence task performance. The findings have implications for the use of motor imagery in rehabilitation protocols. Supplementary Information The online version contains supplementary material available at 10.1007/s00221-022-06389-5.

In modelling the back view, the effect of Angle was significant: RTs to 45,90,135,180,225 and 270 degree rotations were longer compared to the intercept of 0 degrees. Adding Group significantly increased prediction (χ 2 (2) =21.504; p<.001), reflecting shorter RTs in the YA group relative to the OA group intercept, while the PD group did not significantly differ from the OA group. The addition of Sex did not further increase the fit of the model (χ 2 (3) = 2.19; p=.53).
For the palm view, the effect of Angle was again significant: compared to 0 degrees, RTs to 45, 90, 135, and 180 degrees were longer, while RTs to 270 and 315 degree rotations (representing medial orientations) were significantly shorter. The model fit was not significantly increased by the addition of Group (χ 2 (2) =5.01; p=.08) or Sex (χ 2 (2) = 5.81; p=.054).

Accuracy across angles
Accuracy across different rotations generally reflected the pattern for response times, with a similar asymmetrical distribution for the palm view (see Figure S1).
For the back view, there was a significant effect of Angle in the baseline model: compared to the intercept of 0 degrees, accuracy decreased for rotations of 90, 135, 180, 225 and 270 degrees. The addition of Group significantly increased prediction (χ 2 (2) = 6.36; p = .042), reflecting lower accuracy in the PD group, although the main effect within the model did not reach significance at the p<.05 level. Sex did not further increase the fit of the model (χ 2 (3) = .44; p = .93).
For the palm view, the effect of Angle was again significant: compared to the intercept of 0 degrees, accuracy decreased for 90, 135 and 180, but increased for 315 degrees. Group did not significantly increase prediction (χ 2 (2) = .57; p = .75), but a significant effect of Sex (χ 2 (1) = 7.56; p = .006), indicated higher accuracy in males.

Biomechanical constraint effects
RTs and accuracy for medial and lateral orientations in each group are illustrated in Figure S2 and the LMM analysis is summarised in Table S2.
For the back view the effect of Orientation on RT was significant, with shorter RTs to medial than lateral stimuli reflecting a biomechanical constraint effect. The addition of Group increased prediction (χ 2 (4) = 22.64; p<.001), reflecting faster responses in the YA group consistent with the main analysis. Sex also increased prediction (χ 2 (6) = 16.65; p=.011): a three-way interaction between Orientation, Group and Sex indicated a reduced biomechanical constraint effect in the PD group among males. For the palm view, there was again a significant effect of Orientation, with shorter RTs to medial rotations. The fit of the model was increased by the inclusion of Group (χ 2 (4) = 18.19; p=.001), reflecting shorter overall RTs in the YA group, as well as an Orientation*Group interaction, suggesting a smaller biomechanical constraint effect in the YA group. The addition of Sex further increased prediction (χ 2 (6) = 14.78; p=.022): males showed shorter RTs overall, while an interaction of Orientation*Sex indicated a smaller biomechanical constraint effect in males than females.
For stimuli viewed from the back, a biomechanical constraint effect was further indicated by increased accuracy for medial rotations. Prediction was increased by the addition of Group (χ 2 (4) = 11.25; p = .024), reflecting reduced accuracy in the PD group, but there was no significant effect of adding Sex into the model (χ 2 (6) = 5.77; p = .45). For the palm, accuracy was again higher for stimuli in medial rotations. The model was not significantly improved by adding Group (χ 2 (4) = 5.95; p = .20), but was increased by the addition of Sex (χ 2 (2) = 11.58; p = .003), with males showing greater overall accuracy but a reduced advantage for medial rotations, suggesting a smaller effect of biomechanical constraints.

Letter rotation task
The letter rotation task was completed by a subset of 39 participants in the PD group, as well as all 35 OA participants and 30 YA participants, although data from one participant in the YA group was unusable because of a misunderstanding of the task instructions. Performance on the letter rotation task in each group is illustrated in Figure S3 and Table S3.
Similar to the hand task, RTs increased as letters were rotated from 0 to 180 degrees. LMM revealed that RTs were significantly slower for all angles of rotation compared to 0 degrees. Adding Group into the model increased prediction (χ 2 (2) = 23.39; p < .001), reflecting longer RTs in the PD group, but not the YA group, relative to the OA group. Sex also contributed to the model (χ 2 (3) = 11.53; p = .0092), reflecting an interaction between Group and Sex: as evident in Figure 4, this final model revealed that males in the PD group were slower, while females showed similar performance to those in the OA group.
Compared to 0 degrees, accuracy was significantly lower for all rotations except 45 and 315 degrees. Prediction was further increased by the addition of Group (χ 2 (2) = 11.17; p = .0038), with both PD and YA groups being less accurate than the OA group, while Sex did not further affect the fit of the model (χ 2 (1) = .82; p = .36).  Figure S3. Mean RT (upper panel) and accuracy (lower panel) for each angle of rotation in the letter rotation task by group (younger adults, YA; older adults, OA; participants with Parkinson's disease, PD) and sex.